Embodiments of the invention relate generally to electronic converters and, more particularly, to a system for rapid current sensing and optimizing switching timing in a multi-switch power converter.
In power electronics circuits, a half-bridge circuit arrangement is used to control power conversion and current flow through the electronics circuit. FIG. 1 illustrates a known half-bridge circuit topology 2, with the half-bridge circuit 2 including a pair of switches 4, 6 that may be controlled according to a pulse-width modulation (PWM) scheme to convert DC voltage (Vdc) to an AC waveform on the voltage output (vout) to control an AC load such as an AC motor, for example. The half-bridge circuit 2 may be used for one phase of a single- or multi-phase DC-to-DC or DC to AC converter. Typically, switches 4, 6 are operated in an alternating manner in which one switch is in the on-state while the other switch is in the off-state. Controlling which switch is in the on-state via the PWM scheme causes the AC waveform on the voltage output (vout) to be generated according to a desired frequency.
It is recognized that the transition of a switch (e.g., switch 4) from its on-state to its off-state (or its off-state to on-state) is not an instantaneous process. That is, it takes some time for switch 4 to stop conducting current therethrough. If the other switch (e.g., switch 6) begins conducting current prior to the shut-off of current through switch 4, a “shoot-through” condition may be created in which the DC voltage (Vdc) becomes shorted, possibly damaging the voltage source supplying Vdc. Accordingly, in the traditional implementation of half-bridge circuit 2, a dead-time is calculated and added to the PWM scheme to avoid activating both switches 4, 6 to their on-states simultaneously. Furthermore, in a diode-solid state switch series connection, it is recognized that when a switch, such as switch 4, is turned on, there may be a stored charge in the diode that causes the diode to behave as a short circuit or that there may be a residual capacitance across the diode (and an accompanying capacitor) that is discharged. This discharging of current may cause large current spikes that can cause electromagnetic interference (EMI), excess dissipation, and switching loss in the power electronics circuit.
In order to avoid the occurrence of a shoot-through condition and/or a current surge (and associated EMI and switching losses) caused by switching in a half-bridge circuit or diode-solid state switch series connection, current sensing is typically used to control and modulate the gating of the switch or switches in the electronic circuit. Typically, such current sensing is achieved via the use of current sensing circuits such as Hall sensors, low inductance shunts, current sense transformers, etc. However, it is recognized that such current sensing circuits may be costly, inaccurate, and, in some instances, may impact the layout of the power converter. That is, with wide band gap devices, the speed of sensing and control of timing is a critical issue that many implementations fail to achieve. Transition times in such devices are of the order of a few nanoseconds, and sensing is heavily influenced by parasitic elements, and control needs to be fast. To achieve both speed and effective timing control, both the sensing and control circuitry must have minimal delays, both by design as well as by implementation in layout. With specific reference to current sense transformers, it is recognized that the size thereof is the necessary result of the electromagnetic nature of transformers, requiring such sub-components as wire windings, a ferrite or other magnetic core, and the like, and that these physical limitations inhibit the ability of transformers to be miniaturized at the same rate as the solid-state components of power converters, switching power supplies, and other electronic subsystems. Thus, in some implementations that require very tight layouts, such as in electric vehicles that implement power electronics converters such as on-board chargers, traction inverters (and on board distribution at 48V) having wide band gap devices such as SiC and GaN switches, traditional current sensing circuits (including current sense transformers) may negatively impact the layout of the charger/converter, as well as the circuit inductance thereof.
It would therefore be desirable to provide a system and method for current sensing that provides for accurate control and switching in power electronics circuits to minimize shoot through currents, turn-on losses, and EMI. It would also be desirable for such a system and method to provide such current sensing and control with minimal impact on circuit inductance and circuit layout and at a reasonable cost.